Deleting specific array elements

joec_ has asked for the wisdom of the Perl Monks concerning the following question:

Hi, Maybe i'm being exceptionally stupid :P but how does one go about deleting a specific element from an array? I have the following code:

foreach $mol (@molecules) {
if ($mol eq "\n"){
splice (@molecules, scalar($mol),1);
}
}

So, i would like to delete any elements in @molecules (array created via split) that contain elements with just new lines in. When i run it, i get Argument "\n" isn't numeric in splice

Any help or advice appreciated

Thanks -- Joe.

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Re: Deleting specific array elements by salva (Canon) on Nov 26, 2008 at 09:47 UTC
Probably, the easiest way is to use grep: `@molecules = grep { $_ ne "\n" } @molecules;` [download]	[reply] [d/l]
Re^2: Deleting specific array elements by Crian (Curate) on Nov 26, 2008 at 10:00 UTC
If your array is too big to copy it, try something like this: `for my $m (reverse 0 .. $#molecules) { if ($molecules[$m] eq "\n"){ splice (@molecules, $m, 1); } }` [download]	[reply] [d/l]
Re^3: Deleting specific array elements by salva (Canon) on Nov 26, 2008 at 10:47 UTC
That's very inefficient, O(N^2) worst case under the hood. If your data is so big it doesn't fits in memory, it is not a good solution either. Note also, that the grep solution is swap-friendly as it access memory in a sequential manner. Anyway, if you want to minimize memory usage and still be O(N): `my $to = 0; for (@m) { $m[$to++] = $_ if $_ ne "\n"; } splice @m, $to;` [download]	[reply] [d/l]
Re^3: Deleting specific array elements by JavaFan (Canon) on Nov 26, 2008 at 10:47 UTC
Unfortunally, if the array is large, and there's a significant number of elements "\n" in the array, the repeated splicing can be slow. splice() is an `O(n)` operation (with `n` the length of the array), and that's a tight worst case bound (splice may require shifting half the array). If you have a large array, and cannot afford to copy, you can still remove the offending elements in situ: `my $j = 0; for (my $i = 0; $i < @molecules; $i ++) { $molecules[$j++] = $molecules[$i] unless $molecules[$i] eq "\n"; } splice @molecules, $j;` [download]	[reply] [d/l]
Re^4: Deleting specific array elements by matrixmadhan (Beadle) on Nov 26, 2008 at 15:50 UTC
Re^5: Deleting specific array elements by JavaFan (Canon) on Nov 26, 2008 at 17:05 UTC
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Re: Deleting specific array elements by moritz (Cardinal) on Nov 26, 2008 at 09:48 UTC
If `$mol eq "\n"` then `scalar($mol)` is still `\n`, and not a number. But splice expects a number as second (and third) argument. A more idiomatic way to remove newlines is `@molecules = grep { $_ ne "\n" } @molecules;` [download]	[reply] [d/l] [select]
Re: Deleting specific array elements by hexcoder (Curate) on Nov 26, 2008 at 10:09 UTC
If you never need these newline elements, i would avoid storing them in the first place. `@molecules = grep { $_ ne "\n" } split ...` [download]	[reply] [d/l]


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